In the production of steel, the refractory brick linings of conventional process vessels generally wear unevenly due to hot spots, slag attack, or other operating parameters. In order to extend the useful life of such linings, it has become accepted practice to repair excessively worn areas between heats by means of gunning granular refractory materials onto the brick surfaces. That is, a dry mixture including granular refractory grain is pneumatically delivered to the nozzle of a spray gun where it is mixed with water, resulting in a cohesive plastic mass which is projected against the irregularly worn surface, thereby filling in or patching the defective areas. Under the usual circumstances, gunning is initiated within minutes after the molten steel is tapped from the hot vessel. When properly constituted and emplaced, the gunning mix is a relatively stiff plastic mass which adheres to the hot vessel lining and which quickly converts to a relatively hard non-plastic state during the cooling cycle of the furnace. As the temperature of the furnace rises during the next heat, components of the mixture sinter to more permanently bond the refractory gains to themselves and to the brick lining.
The granular refractory material used in gunning mixes is usually composed of a magnesia having a periclase crystalline phase. Such materials include dead burned natural magnesite and a synthetic periclase product derived from seawater or brine. Because of their comparatively lower cost in the market, dead burned dolomite and/or chrom ore grog, along with magnesite in different combinations, have also been conventionally employed as refractory materials in gunning mixes. But due to the lack of any cohesiveness when wetted with water, such refractories must additionally include a binder to provide at least temporary strength to the mass at low temperatures, i.e., prior to subjecting the mass to sintering temperatures. Optionally, a clay mineral is also included in the mixture as a gelling or plasticizing agent.
Both organic and inorganic binders have been suggested for use in gunning mixes. Representative organic binders have included starch, dextrin, various organic sulfonic acids and salts, and tars, pitches and resins, but all of these binders suffer a variety of drawbacks which render them detrimental for use in gunning mixes. For example, refractory mixes containing organic sulfonic acids as a low temperature binder have been found to ignite and burn when applied between heats to the refractory brick lining of a hot (1200.degree. C.) metallurgical vessel, thereby eliminating these materials from consideration as additives for that purpose. For the most part, organic additives contribute excessive porosity to the emplaced material which diminish their refractoriness, durability and strength.
Inorganic binders pose some of the same and some different problems when employed in gunning mixes. For example, sulfuric acid is an excellent binder for magnesia-based refractory materials, reacting very quickly with the MgO content of the mixture to form a strong chemical bond. However, since sulfuric acid is available only in liquid form, it is incapable of being pre-mixed directly with the refractory grain of the gunning mix for ultimate use in conventional maintenance gunning equipment. For this reason, solid inorganic binders capable of being dissolved or dispersed in water have been in common use in gunning mixes, and typical of these are chromic acid, sodium silicate, phosphate glasses, Epsom Salt, and magnesium chloride. As with the organic additives, most of these binding agents suffer one or more disadvantages such as slowness of reaction or air setting, lack of strength at intermediate temperatures, poor slag resistance, etc.
Chromic acid flake is generally accepted to be the best currently available material for bonding periclase grain, due particularly to the mechanical strength, slag resistance, and durability of the resultant products. In gunning mixes, the use of anhydrous chromic acid has been especially beneficial because of the fact that it is a dry solid material which can be pre-mixed with the refractory aggregate conventionally employed in gunning mixes, because it solubilizes rapidly in water when mixed at the nozzle of a conventional gunning device, and because it reacts very quickly with the MgO content of the gunning mix at it is emplaced on the refractory lining surface forming a strong, low temperature bond. Moreover, when the temperature of the process vessel is raised to operating temperatures, the chromic acid residue is totally dispersed throughout the refractory system and aids in the sintering of the refractory grain.
Recently, however, hexavalent chromium has been identified as a potential carcinogen so that the use of anhydrous chromic acid, CrO.sub.3, in the refractory industry has come under close scrutiny. It is, therefore, an object of this invention to provide a dry, solid hexavalent chromium-free binder system for use with magnesia-based refractory mixes, particularly gunning mixes for the repair of metallurgical vessels.